DISCUSSION 391 



iIk' oilici li.iiul. llu' ciR'iny iransfcr plicnomcnon whidi involves exciton 

 iiucraciion clc|)ciul.s on three cliirerent parainelers. One is common to the 

 (Iil<)i()j)h\ Us in anv sitnation. and it is that the intensity of the ai)sorption 

 hand for indivithial molecides deteniiines the energy of the band displace- 

 ment in the aggregate as well as the rate of transfer of the energy. But r)n 

 the other hand, there are two other factors, the distance between the mole- 

 cules and their mutual orientation. It is possible to have in a helical 

 arrangement zero exciton interaction (inifavorable geometry) yet with 

 veiT good electron overlap. The fact that the bands shift very little is no 

 indication that there cannot be good electron overlap because if the mutual 

 transition moments were imfavorably arranged as one runs down the aggre- 

 gate, there would be zero displacement of the spectrum. Therefore, I think 

 the cjuestion is to try to find out by some experimental method whether 

 it is the distance between chlorophylls or whether it is the orientation of 

 adjacent chlorophylls which is giving the relatively small spectral shift in 

 the chloroplast itself. 



